Crystal Device and Method for Manufacturing Crystal Device

ABSTRACT

An object of the present invention is to provide a crystal device that has stable vibration characteristics and that offers high reliability and high accuracy. The crystal device includes a first major face, which contains a portion of a base and a portion of a vibrating prong within a single plane, formed on the crystal plate, and a second major face, which contains another portion of the base and another portion of the vibrating prong within a single plane, formed on a crystal plate, wherein the first major face and the second major face have different outer shapes. The shapes of the first and second major faces can be produced by first forming mask layer patterns on a crystal substrate by exposure through different mask patterns and then etching the crystal substrate using the thus formed mask layer patterns.

FIELD OF INVENTION

The present invention relates to a crystal device comprising a base anda plurality of vibrating prongs protruding from the base, and afabrication method for the same; more particularly, the inventionrelates to a tuning-fork type crystal gyro to be used as an angularvelocity sensor, and a fabrication method for the same.

BACKGROUND OF THE INVENTION

With the widespread use of compact and high-accuracy products such asHDDs (Hard Disk Drives), mobile computers, portable telephones, cartelephones, and other mobile communication apparatus in recent years,there has developed an increasing need to further enhance thereliability and accuracy of crystal devices, typically crystaloscillators, used in these products.

The need for increased reliability and increased accuracy has beengrowing, among others, for crystal gyros used for angular velocitydetection in navigation systems or for camera shake control in videocameras.

FIG. 14 is a diagram showing the structure of a prior art tuning-forktype crystal device.

The structure of the prior art tuning-fork type crystal device will bedescribed below (for example, refer to Patent Document 1). The prior arttuning-fork type crystal device 2 comprises: a tuning-fork-shapedcrystal plate 11 having a base 2 a formed from a crystal and vibratingprongs 2 b protruding the base; and electrodes 501 and 511 forgenerating an electric field across the crystal plate 11. In FIG. 14,the vibrating prongs 2 b each refer to the portion from the tip to thefirst bend of the fork just before the base 2 a thereof, and the base 2a refers to the portion beyond the bend.

A prescribed electric field can be produced across the crystal plate 11by applying different potentials to the respective electrodes 501 and511. Since the crystal plate 11 is a piezoelectric material, when anelectric field is applied across the plate the plate contracts orexpands according to the direction of the electric field. As a result,the vibrating prongs 2 b of the crystal plate 11 vibrate, and thestructure can thus be used as a crystal oscillator 2.

Next, a fabrication method for the prior art tuning-fork type crystaldevice will be described.

FIG. 16 is a diagram showing part of the fabrication process of theprior art tuning-fork type crystal device. First, as shown in FIG. 16A,a crystal substrate 15 is prepared by forming mask layers 21 and 22 andresist layers 31 and 32 one on top of the other on both the upper andlower surfaces of the substrate. Then, the resist layers 31 and 32formed on both surfaces of the crystal substrate 15 are exposed to UVradiation of prescribed wavelength through masks 41 of the same maskpattern 41 a. The resist layers 31 and 32 are exposed only in portionswhere the mask pattern 41 a is not formed.

Next, the resist layers 31 and 32 are developed as shown in FIG. 16B,forming resist layer patterns 31 b and 32 b. Here, the resist layerpatterns 31 b and 32 b are each formed in the same shape as the maskpattern 41 a of the mask 41.

Further, by using the resist layer patterns 31 b and 32 b as the masks,the mask layers 21 and 22 are etched as shown in FIG. 16C, forming masklayer patterns 21 b and 22 b. In this way, the mask layer patterns 21 band 22 b are each formed in the same shape as the mask pattern 41 a ofthe mask 41.

Next, the resist layer patterns 31 b and 32 b are removed as shown inFIG. 16D.

Thereafter, the crystal substrate 15 is etched as shown in FIG. 16E,thus producing the outer shape of the crystal plate 11.

FIG. 17 is a diagram showing the shape of the crystal plate formed bythe prior art etching and the shapes of the mask layer patterns forcomparison.

As shown in FIG. 17, the major faces 211 and 221 of the crystal plate 11formed by the etching in FIG. 16E, which were the plane surfaces of thecrystal substrate 15, are formed in substantially the same shapes as therespective mask layer patterns 21 b and 22 b. Since the mask layers 21 band 22 b are both substantially the same in shape as the mask pattern 41a, it follows that the two major faces 211 and 221 of the crystal plate11 are both formed in substantially the same shape.

After the etching in FIG. 16E, the mask layer patterns 21 b and 22 b areremoved, and the electrodes 501 and 511 are formed, completing thefabrication of the tuning-fork type crystal device 2 shown in FIG. 14.

The structure and the fabrication method of the tuning-fork type crystaldevice according to the prior art described above are used for crystalresonators, crystal oscillators, crystal gyros, and various otherapplications.

In the prior art tuning-fork type crystal device, it is common practiceto form the crystal plate 11 by etching. However, since the crystal hasthe property that its reaction rate differs depending on the directionit is etched (this property is generally called the anisotropic etchingproperty), a ridge-like unetched portion 111 projecting from a side faceis necessarily formed, as shown in FIG. 14, in an intermediate region ofthe base 2 a connecting between the vibrating prongs 2 b, that is, inFIG. 14, the portion that originates from the tip of one vibrating prong2 b, passes through the root of the one vibrating prong 2 b and throughthe root of the other vibrating prong 2 b, and leads to the tip of thatother vibrating prong 2 b (this portion is generally referred to as thecrotch portion).

When the unetched portion 111 is formed, as shown in FIG. 14, thevibrating prong 2 b which should normally vibrate in the X-axisdirection vibrates in a direction slightly tilted toward the Z-axisdirection as indicated by W1. The vibration W1 produced by tiltingtoward the Z-axis direction tends to tilt greater toward the Z-axisdirection as the unetched portion 111 becomes larger.

FIG. 15 is a diagram showing an enlarged view and a side view of thecrotch portion of the tuning fork and its adjacent regions in the priorart tuning-fork type crystal device. Here, FIG. 15(b) shows the shape ofthe side face as viewed from the direction C in FIG. 15(a). As shown inFIG. 15(b), the unetched portion 111 is formed on the side face of thecrystal plate 11 in the shape of a ridge extending obliquely from onemajor face 211 to the other major face 221 of the crystal plate 11. Thisproduces the same effect as if a prop were placed obliquely across theroot portion of the vibrating prong 2 b and, with this effect, thedirection of vibration is tilted toward the Z-axis direction, producingthe vibration W1. In this patent specification, the root refers to theportion at the boundary between the vibrating prong 2 b and the base 2a; in FIG. 15(a), the bend corresponds to the root.

Further, as shown in FIG. 15(a), the unetched portion 111 left after theetching is formed only on one vibrating prong side of the crotch portionof the tuning fork, and as a result, the left and right vibrating prongs2 b vibrate in different directions, as shown in FIG. 14.

When the direction of vibration of any one vibrating prong 2 b isunstable as described above, the resulting crystal device is oftenunstable and inaccurate in operation; in the prior art, therefore, thebalance between the directions of vibration has been adjusted afterforming the crystal plate 11, by performing an additional processingstep for appropriately removing portions of the electrodes 501 and 511and the crystal plate 11.

One proposal has been made in Patent Document 1 to stabilize thevibration of the crystal device. The proposal made in Patent document 1aims to stabilize the vibration of the crystal device by optimizing theplan shape of the crystal plate 11 according to Patent Document 1, it isstated that the plan shape of one major face 201 is the same as the planshape of the other major face 211.

However, according to experiments conducted by the present inventors, ithas been confirmed that, as long as the plan shape of one major face 201of the crystal plate 11 is the same as the plan shape of the other majorface 211, no appreciable change occurs in the shape of the unetchedportion 111 and, in most cases, the unetched portion 111 which causesthe unstable vibration is formed in substantially the same shape, thoughits size may vary somewhat. That is, to whatever shape the mask pattern41 a of the mask 41 alone is optimized that determines the plan shape ofthe crotch portion of the tuning fork, as shown in Patent Document 1,the unetched portion 111 which causes the unstable vibration isnecessarily formed as long as the mask pattern 41 a of the same shape isused for both the major faces 201 and 211.

In the case of a crystal gyro as one application example of thetuning-fork type crystal device, the vibration tilted toward the Z axissuch as the vibration W1 shown in FIG. 14 becomes a very seriousproblem. The reason is that, in the crystal gyro, the Z-axis directionis nothing but the direction of the angular velocity to be detected, andany vibration in the Z-axis direction, such as the vibration W1, canaffect the accurate detection of the angular velocity. As a result, theprior art crystal gyro has had the problem of low accuracy and lowreliability.

Patent Document 1: Japanese Unexamined Patent Publication No. H10-41772(page 2, FIGS. 2 and 3)

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a crystal device anda method for manufacturing the same that can overcome the prior artproblem.

It is another object of the present invention to provide a high-accuracyand high-reliability crystal device and a method for manufacturing thesame.

It is a further object of the present invention to provide ahigh-accuracy and high-reliability crystal gyro that can accuratelydetect angular velocity by using the crystal device of the invention inthe crystal gyro, and a method for manufacturing such a crystal gyro.

A crystal device according to the present invention has a crystal platethat includes a base and a plurality of vibrating prongs protruding fromthe base, wherein two major faces, each containing the base and theplurality of vibrating prongs, are formed on the crystal plate, andwherein the two major faces have respectively different outer shapes.

A crystal device according to the present invention has a first majorface, which contains a portion of the base and a portion of thevibrating prong within a single plane, and a second major face, whichcontains another portion of the base and another portion of thevibrating prong within a single plane, are formed on the crystal plate,wherein the first major face and the second major face have differentouter shapes.

Preferably, in the crystal device according to the present invention,the crystal plate includes at least a first vibrating prong and a secondvibrating prong. Further preferably, the first and second major faceseach have a crotch portion defined by a line that originates from thetip of the first vibrating prong, passes through the root of the firstvibrating prong and through the root of the second vibrating prong, andleads to the tip of the second vibrating prong, wherein the crotchportion of the first major face and the crotch portion of the secondmajor face have different shapes.

Preferably, in the crystal device according to the present invention,the number of bends contained in the crotch portion of the first majorface is different from the number of bends contained in the crotchportion of the second major face.

Preferably, in the crystal device according to the present invention,the angle of a bend contained in the crotch portion of the first majorface is different from the angle of a bend contained in the crotchportion of the second major face.

Preferably, in the crystal device according to the present invention,the number of center points representing curved sections contained inthe crotch portion of the first major face is different from the numberof center points representing curved sections contained in the crotchportion of the second major face.

Preferably, in the crystal device according to the present invention,the curvature of a curved section contained in the crotch portion of thefirst major face is different from the curvature of a curved sectioncontained in the crotch portion of the second major face.

A method for manufacturing a crystal device according to the presentinvention, the crystal device having a crystal plate that includes abase and a plurality of vibrating prongs protruding from the base,includes the steps of forming a resist layer made of a photosensitivematerial and a mask layer having corrosion resistance to etching of acrystal on each of two plane surfaces of a crystal substrate, exposingthe resist layer formed on one plane surface of the crystal substrate toradiation through a first mask on which a first mask pattern of aprescribed shape is drawn and exposing the resist layer formed on theother plane surface of the crystal substrate to radiation through asecond mask on which a second mask pattern is drawn that differs inshape from the first mask pattern, forming a first resist layer patternby patterning the resist layer on the one plane surface of the crystalsubstrate into a shape corresponding to the first mask pattern, andforming a second resist layer pattern by patterning the resist layer onthe other plane surface of the crystal substrate into a shapecorresponding to the second mask pattern, forming a first mask layerpattern by patterning the mask layer on the one plane surface of thecrystal substrate into the shape corresponding to the first maskpattern, and forming a second mask layer pattern by patterning the masklayer on the other plane surface of the crystal substrate into the shapecorresponding to the second mask pattern, and forming the crystal platein a desired shape by etching the crystal substrate using the first maskpattern and the second mask pattern as masks.

The method according to the present invention includes the steps offorming a resist layer made of a photosensitive material and a masklayer having corrosion resistance on each of two plane surfaces of acrystal substrate, exposing the resist layer formed on the first planesurface of the crystal substrate to radiation through a first mask onwhich a first mask pattern is drawn and exposing the resist layer formedon the second plane surface of the crystal substrate to radiationthrough a second mask on which a second mask pattern is drawn thatdiffers in shape from the first mask pattern, forming a first resistlayer pattern by patterning the resist layer on the first plane surfaceof the crystal substrate into a shape corresponding to the first maskpattern and forming a second resist layer pattern by patterning theresist layer on the second plane surface of the crystal substrate into ashape corresponding to the second mask pattern, forming a first masklayer pattern by patterning the mask layer on the first plane surface ofthe crystal substrate into a shape corresponding to the first maskpattern and forming a second mask layer pattern by patterning the masklayer on the second plane surface of the crystal substrate into a shapecorresponding to the second mask pattern, and forming the crystal plateby etching the crystal substrate through the first mask layer patternand the second mask layer pattern.

Preferably in the method according to the present invention, the firstmask layer pattern has an outer shape that contains a portion of thebase and a portion of the vibrating prong within a single plane, and thesecond mask layer pattern has an outer shape that contains anotherportion of the base and another portion of the vibrating prong within asingle plane.

Preferably, in the method according to the present invention, thecrystal plate includes at least a first vibrating prong and a secondvibrating prong.

Preferably, in the method according to the present invention, the firstand second mask layer patterns each have a crotch portion defined by aline that originates from the tip of the first vibrating prong, passesthrough the root of the first vibrating prong and through the root ofthe second vibrating prong, and leads to the tip of the second vibratingprong.

Preferably, in the method according to the present invention, the crotchportion of the first mask layer pattern and the crotch portion of thesecond mask layer pattern have different shapes.

Preferably, in the method according to the present invention, the numberof bends contained in the crotch portion of the first mask layer patternis different from the number of bends contained in the crotch portion ofthe second mask layer pattern.

Preferably, in the method according to the present invention, the angleof a bend contained in the crotch portion of the first mask layerpattern is different from the angle of a bend contained in the crotchportion of the second mask layer pattern.

Preferably, in the method according to the present invention, the numberof center points representing curved sections contained in the crotchportion of the first mask layer pattern is different from the number ofcenter points representing curved sections contained in the crotchportion of the second mask layer pattern.

Preferably, in the method according to the present invention, thecurvature of a curved section contained in the crotch portion of thefirst mask layer pattern is different from the curvature of a curvedsection contained in the crotch portion of the second mask layerpattern.

One feature of the crystal device according to the present invention isthat the two major faces of the crystal plate have different outershapes, in particular, the crotch portions of the respective major faceshave different shapes.

According to the crystal device of the present invention, the vibrationof the (tuning-fork type) crystal plate can be stabilized, and as aresult, a crystal device having high reliability and high accuracy canbe achieved.

Further, according to the crystal device of the present invention, theprocessing step that has had to be performed after the etching step inthe prior art in order to achieve stable vibration characteristics canbe simplified or omitted, and as a result, productivity of the crystaldevice can be increased compared with the prior art.

Furthermore, when the crystal device of the present invention is usedfor a crystal gyro, the vibration can be stabilized, and therefore, theangular velocity can be detected accurately; as a result, a crystal gyrocan be obtained that have high detection sensitivity and high accuracy.

According to the crystal device manufacturing method of the presentinvention, a crystal device having stable vibration characteristics canbe formed, and as a result, a crystal device having high reliability andhigh accuracy can be produced.

Furthermore, according to the crystal device manufacturing method of thepresent invention, the processing step that has had to be performedafter the etching step in the prior art in order to achieve stablevibration characteristics can be simplified or omitted, and as a result,productivity can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the structure of a crystal device 1according to the present invention.

FIG. 2 is a diagram showing an enlarged view and a side view of a crotchportion and its adjacent regions in the crystal device 1 according tothe present invention.

FIG. 3 is a diagram showing the first half of the manufacturing processof the crystal device 1 according to the present invention.

FIG. 4 is a diagram showing the second half of the manufacturing processof the crystal device 1 according to the present invention.

FIG. 5 is a diagram showing the shape of a crystal plate 10 and theshapes of mask layer patterns in the crystal device 1 according to thepresent invention.

FIG. 6 is a diagram showing the etching characteristic of a crystalsubstrate formed from a conventional synthetic crystal.

FIG. 7 is a diagram showing mask patterns used in the manufacturingprocess of the crystal device 1 according to the present invention.

FIG. 8 is a diagram showing mask patterns used in the manufacturingmethod for an alternative crystal device 2 according to the presentinvention.

FIG. 9 is a diagram showing the shape of a crystal plate 12 and theshapes of mask layer patterns in the alternative crystal device 2according to the present invention.

FIG. 10 is a diagram showing mask patterns used in the fabricationmethod for a further alternative crystal device 3 according to thepresent invention.

FIG. 11 is a diagram showing the shape of a crystal plate 13 and theshapes of mask layer patterns in the further alternative crystal device3 according to the present invention.

FIG. 12 is a diagram showing mask patterns used in the manufacturingmethod for a still further alternative crystal device 4 according to thepresent invention.

FIG. 13 is a diagram showing the shape of a crystal plate 14 and theshapes of mask layer patterns in the still further alternative crystaldevice 4 according to the present invention.

FIG. 14 is a diagram showing the structure of a prior art tuning-forktype crystal device.

FIG. 15 is a diagram showing an enlarged view and a side view of thecrotch portion of a tuning fork and its adjacent regions in the priorart tuning-fork type crystal device.

FIG. 16 is a diagram showing part of the manufacturing process of theprior art tuning-fork type crystal device.

FIG. 17 is a diagram showing the shape of the crystal plate formed byetching according to the prior art and the shapes of mask layer patternsfor comparison.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a diagram showing the structure of a crystal device 1according to the present invention.

The crystal device 1 according to the present invention comprises acrystal plate 10 formed from a crystal, and electrodes 500 and 510. Thecrystal plate 10 is formed in the shape of a tuning fork which is madeup of a base 1 a and a plurality of vibrating prongs 1 b protruding fromthe base 1 a. The electrodes 500 and 510 are electrically separated fromeach other, and an electric field can be produced across the crystalplate 10 by applying different potentials to the respective electrodes.Since the crystal plate 10 is a piezoelectric material, the platecontracts or expands according to the direction of the applied electricfield; this contracting/expanding motion causes the vibrating prongs 1 bto vibrate. When a voltage is applied between the electrodes 500 and 510of the crystal device 1, the two vibrating prongs 1 b produce vibrationsW0 along the X-axis direction.

In this patent specification, the portion defined by an outlineextending from the tip to the first bend in the direction of the basewill hereinafter be referred to as the “vibrating prong”, the portionbeyond the bend will be referred to as the “base”, and the portion atthe boundary between the base and the vibrating prong will be referredto as the “root”.

The difference between the crystal device 1 of the present inventionshown in FIG. 1 and the tuning-fork type crystal device of the prior artlies in the outer shape of the crystal plate 10, especially in theshapes of the two major faces 210 and 220 formed by the plane faces ofthe base 1 a and vibrating prongs 1 b of the crystal plate 10. In theembodiment of the present invention, the shape of the major face 210 isdifferent from the shape of the major face 220, especially in theintermediate region connecting between the two vibrating prongs (thatis, in FIG. 1, the portion that originates from the tip of one vibratingprong 1 b, passes through the root of the one vibrating prong 1 b andthrough the root of the other vibrating prong 1 b, and leads to the tipof that other vibrating prong 1 b (this portion is hereinafter referredto as the “crotch portion”)). This constitutes one of the most importantfeatures of the present invention.

That is, while the shape of the crotch portion of the major face 210contains three bends, the shape of the crotch portion of the major face220 contains two bends (one of the two bends of the major face 220 ishidden from view in FIG. 1). In any prior art tuning-fork type crystaldevice, the two major faces 210 and 220 are not formed so as to havedifferently shaped crotch portions as described above, but are alwaysformed in the same shape.

When the crotch portions of the two major faces 210 and 220 of thecrystal plate 10 are formed in different shapes, the collateral effectis that the shape of the crystal plate (the shape of the unetchedportion 110 formed on a side face of the crotch portion of the crystalplate 10) can be made different from that of the prior art. In thecrystal device according to the present embodiment, stable vibrationcharacteristics such as the vibrations W0 along the X axis shown in FIG.1 can be obtained by forming the unetched portion 110 in an optimumshape.

FIG. 2A is an enlarged view of the crotch portion and its adjacentregions in the tuning-fork type crystal device 1 shown in FIG. 1, andFIG. 2B is a diagram showing the shape of the side face as viewed fromthe direction B in FIG. 2A.

As shown in FIGS. 2A and 2B, the unetched portion 110 of the crystaldevice 1, which is left on the side face of the crotch portion of thecrystal plate 10, is formed as a ridge-like protrusion having a ridgeline substantially parallel to the two major faces 210 and 220. That is,the shape of the unetched portion 110 of the crystal device 1 isdifferent from that of the prior art which has a ridge line extendingobliquely between the two major faces. Furthermore, the size of theunetched portion 110 can be reduced (to about one quarter in terms ofvolume ratio) compared with the prior art (in which the same maskpattern 21 b is used for both the major faces).

As described above, according to the present invention, the unetchedportion 110 formed on the side face of the crotch portion of the crystalplate 10 can be made very small and can be formed in the shape of aprotrusion having a ridge line substantially parallel to the two majorfaces 210 and 220. This, therefore, does not provide any straining propto the root portion of the vibrating prong, and the vibrating prong canproduce stable vibrations W0 along the X axis as designed.

A manufacturing method for the crystal device 1 will be described below.

FIG. 3 is a diagram showing the first half of the manufacturing processof the crystal device 1, FIG. 4 is a diagram showing the second half ofthe manufacturing process of the crystal device 1, and FIG. 7 is adiagram showing mask patterns used in the manufacturing process of thecrystal device 1.

First, as shown in FIG. 3A, a crystal substrate 15 is prepared byforming mask layers 21 and 22 and resist layers 31 and 32 one on top ofthe other on the upper and lower plane surfaces of the substrate. Thecrystal substrate 15 used in the present embodiment is a substrategenerally called a Z plate whose plane faces are substantiallyperpendicular to the crystallographic z-axis direction of the crystal(the plane faces make an angle of 85 to 95 degrees with respect to the Zaxis).

Further, in the present embodiment, the mask layers 21 and 22 are eachformed from a composite layer consisting of a chromium (Cr) filmdeposited to a thickness of 0.05 μm on the crystal substrate 15 and agold (Au) film deposited to a thickness of 0.15 μm on the Cr film. Theresist layers 31 and 32 are formed using a positive resist OFPR-800manufactured by Tokyo Ohka Kogyo Co., Ltd.

One plane surface of the crystal substrate 15 on which the mask layer 21and the resist layer 31 are formed and the other plane surface on whichthe mask layer 22 and the resist layer 32 are formed are exposed to UVradiation of prescribed wavelength through masks 41 and 42,respectively.

In the present embodiment, the mask pattern 41 a drawn on the mask 41and the mask pattern 42 a drawn on the mask 42 are different in shape.The mask pattern 41 a drawn on the mask 41 has an outer shape definingthe plan shape of the base and the plan shapes of the two vibratingprongs, as shown in FIG. 7A. In the mask pattern 41 a, the intermediateregion (i.e., the crotch portion) connecting between the two vibratingprongs has a shape that contains three bends formed by bending astraight line. On the other hand, the mask pattern 42 a formed on themask 42 has an outer shape defining the plan shape of the base and theplan shapes of the two vibrating prongs, as shown in FIG. 7B. In themask pattern 42 a, the intermediate region (i.e., the crotch portion)connecting between the two vibrating prongs has a shape that containstwo bends formed by bending a straight line. In this way, the shape ofthe crotch portion differs between the mask pattern 41 a of the mask 41and the mask pattern 42 a of the mask 42; in particular, the number ofbends contained in the crotch portion differs between them.

Next, the resist layers 31 and 32 are developed as shown in FIG. 3B,forming resist layer patterns 31 a and 32 a. In the present embodiment,the resist layers are developed using a developer specially prepared forOFPR-800.

Here, the resist layer pattern 31 a is formed in the same shape as themask pattern 41 a of the mask 41, while the resist layer pattern 32 a isformed in the same shape as the mask pattern 42 a of the mask 42. Thatis, the resist layer pattern 31 a has an outer shape defining the planshape of the base and the plan shapes of the two vibrating prongs, andthe crotch portion connecting between the two vibrating prongs in theresist layer pattern 31 a has a shape that contains three bends. On theother hand, the resist layer pattern 32 a has an outer shape definingthe plan shape of the base and the plan shapes of the two vibratingprongs, and the crotch portion connecting between the two vibratingprongs in the resist layer pattern 32 a has a shape that contains twobends.

Next, using the resist layer patterns 31 a and 32 a as the masks, themask layers 21 and 22 are etched as shown in FIG. 3C, forming mask layerpatterns 21 a and 22 a. In the present embodiment, the mask layerpatterns 21 a and 22 a are formed by etching the Au film in aqua regiaand the Cr film in an etching solution of nitric acid.

Here, the mask layer pattern 21 a, like the resist layer pattern 31 a,is formed in the same shape as the mask pattern 41 a of the mask 41,while the mask layer pattern 22 a, like the resist layer pattern 32 a,is formed in the same shape as the mask pattern 42 a of the mask 42.That is, the mask layer pattern 21 a has an outer shape defining theplan shape of the base and the plan shapes of the two vibrating prongs,and the crotch portion connecting between the two vibrating prongs inthe mask layer pattern 21 a has a shape that contains three bends. Onthe other hand, the mask layer pattern 22 a has an outer shape definingthe plan shape of the base and the plan shapes of the two vibratingprongs, and the crotch portion connecting between the two vibratingprongs in the mask layer pattern 22 a has a shape that contains twobends.

Next, by removing the resist layer patterns 31 a and 32 a, as shown inFIG. 3D, the mask layer patterns 21 a and 22 a are formed in differentshapes on the respective plane surfaces of the crystal substrate 15. Inthe present embodiment, OFPR-800 used for forming the resist layerpatterns 31 a and 32 a is removed using acetone.

FIG. 4A is a diagram showing the mask layer patterns 21 a and 22 aformed in different shapes, as described above, on the plane surfaces ofthe crystal substrate 15. The A-A cross section of FIG. 4A is shown inFIG. 3D. By etching the crystal substrate 15 shown in FIG. 4A, thecrystal plate 10 whose outer shape is defined by the mask layers 21 aand 22 a, and whose one major face is differently shaped than its othermajor face, is completed as shown in FIG. 4B.

FIG. 5 is a diagram showing the shape of the crystal plate 10 used inthe crystal device 1 according to the present invention and the shapesof the mask layer patterns for comparison.

In the present embodiment, one major face 210 of the crystal plate 10 isformed in substantially the same shape as the mask layer pattern 21 a,and has an outer shape defined by the plan shape of the base and theplan shapes of the two vibrating prongs, and the crotch portionconnecting between the two vibrating prongs has a shape that containsthree bends. On the other hand, the other major face 220 of the crystalplate 10 is formed in substantially the same shape as the mask layerpattern 22 a, and has an outer shape defined by the plan shape of thebase and the plan shapes of the two vibrating prongs, and the crotchportion connecting between the two vibrating prongs has a shape thatcontains two bends. Accordingly, in the present embodiment, the shape ofthe one major face 210 of the crystal plate 10 is different from that ofthe other major face 220.

The crystal plate 10 formed by etching in FIG. 4B has an unetchedportion 110 left unetched on the side face of the crotch portionconnecting between the two vibrating prongs. This is due to the etchingcharacteristic peculiar to the crystal. The crystal has thecharacteristic that its etch rate varies with its crystal orientation;this characteristic is generally known as the anisotropic etchingcharacteristic. A protrusion such as the unetched portion 110 isnecessarily formed on the side face of the crotch portion of the crystalplate 10 because of the effect of the anisotropic etching characteristicof the crystal.

In the present embodiment, however, the size of the unetched portion 110can be greatly reduced compared with the prior art, because the shapesof the mask layer patterns 21 a and 22 a are determined by consideringthe anisotropic etching of the respective major faces, that is, forportions where the etch rate is slow, the patterns are formed withlarger aperture in order to make such portions easier to etch but, forportions where the etch rate is fast, the patterns are formed withsmaller aperture in order to make it difficult for the etching toproceed. Furthermore, in the present embodiment, the unetched portion110 can be formed in a different shape than that of the prior art, thatis, in the shape of a ridge-like protrusion having a ridge linesubstantially parallel to the two major faces 210 and 220. This is alsothe effect of the shapes of the mask layer patterns 21 a and 22 a.

In this way, the shapes of the mask layer patterns 21 a and 22 a aredifferently designed for the respective major faces by considering theanisotropic etching of the crystal. This also constitutes a feature ofthe present invention.

Through many experiments conducted by the present inventors, severalcombinations have been obtained for the optimum shapes of the mask layerpatterns 21 a and 22 a. As a result, it has been found that the optimumshapes of the mask layer patterns 21 a and 22 a are not the same for thetwo major faces, but that the best results can be obtained whendifferent shapes are used for the respective major faces.

Finally, the mask layer patterns 21 a and 22 a are removed, and theelectrodes 500 and 511 are formed, to complete the fabrication of thetuning-fork type crystal device as shown in FIG. 1.

It has also been verified that the tuning-fork type crystal device 1according to the present invention can be very effectively used forcrystal gyros. In crystal gyros that utilize Coriolis forces, the Z-axisdirection (the direction parallel to the Z axis which is one of thecrystallographic axes of the crystal) is perpendicular to the vibrationdirection of the vibrating prongs, and is nothing but the direction ofthe angular velocity to be detected. In the tuning-fork type crystaldevice 1, as can be seen in the vibrations W0, hardly any vibrationsoccur in the Z-axis direction, so that only the component of the angularvelocity to be detected can be accurately detected. As a result, whenthe tuning-fork type crystal device 1 according to the present inventionis used, a crystal gyro can be achieved that has very good detectionsensitivity and high reliability compared with the prior art.

Usually, in crystal gyros, the direction of the angular velocity to bedetected is perpendicular to the vibration direction of the vibratingprongs, and the angular velocity cannot be detected accurately unlessthe vibration direction is stable as designed. With the crystal gyrousing the crystal device of the present invention, on the other hand,the angular velocity perpendicular to the vibration direction can beaccurately detected, because the vibration direction is stable asdesigned.

Next, a description will be given of the unetched portion.

As earlier described, in the crystal device 1 of the present invention,since the outer shapes of the two major faces of the crystal plate aremade different from each other, the unetched portion remaining on theside face of the crotch portion of the crystal plate can be made verysmall. Furthermore, the unetched portion can be formed in the shape of aprotrusion having a ridge line substantially parallel to the two majorfaces. Moreover, the unetched portion can be formed parallel to themajor faces of the crystal plate in such a manner as to be substantiallycentralized on the side face of the crotch portion. In this way, theunetched portion whose shape is significantly different from that of theprior art constitutes a feature of the present invention.

In the crystal plate used for the prior art crystal device, the unetchedportion 111 was formed in a ridge-like shape on the side face of thecrotch portion in such a manner as to extend across the side face of thecrystal plate obliquely from one major face to the other major face (seeFIG. 15B). This produced the same effect as if a prop had been placedobliquely across the root portion of the vibrating prong, causing thevibrating prong to vibrate in a direction tilted toward the Z-axisdirection.

On the other hand, in the crystal device 1 of the present invention,since the unetched portion 110 is small in size, and is formed parallelto the major faces of the crystal plate in such a manner as to besubstantially centralized on the side face of the crotch portion, asdescribed above, the root portion of the vibrating prong is notphysically strained, and the desired stable vibrations can be produced.

The unetched portion 110 is formed when the crystal substrate is etched.The shape of the unetched portion 110 is substantially determined by theshapes of the mask layer patterns formed in the step that precedes theetching step. The shape of the unetched portion 110 in the crystaldevice 1 of the present invention is achieved by making the shape of themask layer pattern different between the upper and lower surfaces of thecrystal substrate. This utilizes the etching characteristic that theetch rate differs between the upper and lower surfaces of the crystalsubstrate.

FIG. 6 is a diagram showing the etching characteristic of the crystalsubstrate formed from a conventional synthetic crystal.

The crystal substrates 15 shown in FIGS. 6A and 6B are each a substratecut so as to have plane faces substantially perpendicular to thecrystallographic z axis (the thus cut substrate is generally called a “Zplate”). However, the growth direction “g” of the crystal in the crystalsubstrate of FIG. 6A is opposite to the growth direction “g” of thecrystal in the crystal substrate of FIG. 6B. That is, FIGS. 6A and 6Brepresent the relationship between the front and back of the crystalsubstrate.

Usually, in the crystal substrate 15 (Z plate) cut so as to have planefaces perpendicular to the z axis as shown in FIG. 6A or 6B, there arethree Y axes (Y₁₋₁ to Y₁₋₃ or Y₂₋₁ to Y₂₋₃) spaced 120 degrees apartfrom each other on each plane face.

If such crystal substrates 15 are etched using the same conditions, theetch rates e1 in directions parallel to the Y axes and the etch rate e2in directions normal to the Y axes are different between the case ofFIG. 6A and the case of FIG. 6B.

Generally, in the plane face toward which the growth proceeds in thedirection “g” as shown in FIG. 6A, the etch rate e1 in the directionparallel to the Y axis (Y₁₋₁) and the etch rate e2 in the directionnormal to the Y axis (Y₁₋₂) have the relationship defined by e1>e2. Onthe other hand, in the plane face from which the growth proceeds in thedirection “g” as shown in FIG. 6B, the etch rate e1 in the directionparallel to the Y axis (Y₂₋₁) and the etch rate e2 in the directionnormal to the Y axis (Y₂₋₂) have the relationship defined by e1<e2.

If the shapes of the mask layer patterns are designed by consideringthese etch rates e1 and e2, it becomes possible to form the crystalplate having the unetched portion as depicted in the present invention.

For example, suppose that the upper face 210 of the crystal plate 10shown in FIG. 5 is formed as shown in FIG. 6A, and that the three Y axes(Y₁₋₁ to Y₁₋₃) in FIG. 6A are arranged as shown in FIG. 5. In this case,the direction e2 at right angles to the Y₁₋₂ axis substantiallycoincides with the projecting direction of the unetched portion 110.Since the etching is difficult to proceed in the direction e2, theunetched portion is formed in the direction e2.

FIG. 8 is a diagram showing a combination of mask patterns for analternative crystal device 2 according to the present invention.

In the case of the alternative crystal device 2 according to the presentinvention, the tuning-fork type crystal device 2 is fabricated by usingmask patterns 41 b and 42 b having different bend angles (θ1 and θ2) asshown in FIGS. 8A and 8B. The fabrication method of the crystal device 2is the same as that shown in FIG. 3, except that the mask patterns arechanged to the mask patterns 41 b and 42 b, and therefore, thedescription of the method will not be repeated here.

FIG. 9 is a diagram showing the shape of the crystal plate 12 used inthe tuning-fork type crystal device 2 according to the present inventionand the shapes of the mask layer patterns for comparison.

By using the mask patterns 41 b and 42 b that differ in the bend angleof the crotch portion, the crystal plate 12 can be fabricated that hastwo major faces that differ in the bend angle of the crotch portion. Theunetched portion 112 left on the thus formed crystal plate 12 is shapedin the form of a protrusion having a ridge line substantially parallelto the two major faces. That is, the shape of the unetched portion 112is different from that of the prior art which has a ridge line extendingobliquely between the two major faces. Furthermore, the size of theunetched portion 112 can be reduced (to about four-ninths in terms ofvolume ratio) compared with the prior art (in which the same maskpattern 41 b is used for both the major faces).

As a result, in the present embodiment also, there is no straining propin the root portion of the vibrating prong, and the vibrating prong canproduce stable vibrations along the X axis as designed, thus achievingthe crystal device having high reliability and high accuracy.

FIG. 10 is a diagram showing a combination of mask patterns for afurther alternative crystal device 3 according to the present invention.

In the further alternative crystal device 3 according to the presentinvention, the tuning-fork type crystal device 3 is fabricating usingmask patterns 41 c and 42 c that differ in the number of curved sections(R1, R2, R3) forming the crotch portion, as shown in FIGS. 10A and 10B.The fabrication method of the crystal device 3 is the same as that shownin FIG. 3, except that the mask patterns are changed to the maskpatterns 41 c and 42 c, and therefore the description of the method willnot be repeated here.

FIG. 11 is a diagram showing the shape of the crystal plate 13 used inthe tuning-fork type crystal device 3 according to the present inventionand the shapes of the mask layer patterns for comparison.

By using the mask patterns 41 c and 42 c that differ in the number ofcurved sections forming the crotch portion, the tuning-fork type crystaldevice 3 can be fabricated that has two major faces that differ in thenumber of curved sections forming the crotch portion. The unetchedportion 113 left on the thus formed crystal plate 13 is shaped in theform of a protrusion having a ridge line substantially parallel to thetwo major faces. That is, the shape of the unetched portion 113 isdifferent from that of the prior art which has a ridge line extendingobliquely between the two major faces. Furthermore, the size of theunetched portion 113 can be reduced (to about one quarter in terms ofvolume ratio) compared with the prior art (in which the same maskpattern 41 c is used for both the major faces).

As a result, in the present embodiment also, there is no straining propin the root portion of the vibrating prong, and the vibrating prong canproduce stable vibrations along the X axis as designed, thus achievingthe crystal device having high reliability and high accuracy. Here, inthe present invention, it is to be understood that the number of curvedsections forming the crotch portion is represented by the number ofcenter points of the curved sections, and that a continuous curvehaving, for example, two center points is regarded as containing twocurved sections.

FIG. 12 is a diagram showing a combination of mask patterns for a stillfurther alternative crystal device 4 according to the present invention.

In the still further alternative crystal device 4 according to thepresent invention, the tuning-fork type crystal device 4 is fabricatingusing mask patterns 41 d and 42 d that differ in the curvature of thecurve (R1, R2) forming the crotch portion, as shown in FIGS. 12A and12B. The fabrication method of the crystal device 4 is the same as thatshown in FIG. 3, except that the mask patterns are changed to the maskpatterns 41 d and 42 d, and therefore the description of the method willnot be repeated here.

FIG. 13 is a diagram showing the shape of the crystal plate 14 used inthe tuning-fork type crystal device 4 according to the present inventionand the shapes of the mask layer patterns for comparison.

By using the mask patterns 41 d and 42 d that differ in the curvature ofthe curve forming the crotch portion, the tuning-fork type crystaldevice 4 can be fabricated that has two major faces that differ in thecurvature of the curve forming the crotch portion. The unetched portion114 left on the thus formed crystal plate 14 is shaped in the form of aprotrusion having a ridge line substantially parallel to the two majorfaces. That is, the shape of the unetched portion 114 is different fromthat of the prior art which has a ridge line extending obliquely betweenthe two major faces. Furthermore, the size of the unetched portion 114can be reduced (to about four-ninths in terms of volume ratio) comparedwith the prior art (in which the same mask pattern 41 d is used for boththe major faces).

As a result, in the present embodiment also, there is no straining propin the root portion of the vibrating prong, and the vibrating prong canproduce stable vibrations along the X axis as designed, thus achievingthe crystal device having high reliability and high accuracy.

As described above, according to the present invention, the crystaldevices 1 to 4 having high reliability and high accuracy can beachieved. When such crystal devices 1 to 4 are used for products such ascrystal resonators and crystal oscillators, high reliability andaccuracy can be obtained. Furthermore, when such crystal devices 1 to 4are used for crystal gyros, the angular velocity can be detectedaccurately, and crystal gyros can be obtained that have high detectionsensitivity and high accuracy.

Further, in the crystal devices 1 to 4 described above, not only can thesize of the unetched portion be reduced compared with that of the priorart, but the unetched portion can be formed in the shape of a protrusionhaving a ridge line substantially parallel to the two major faces.Accordingly, in the crystal devices 1 to 4, the processing stepperformed after the etching step in the prior art in order to achievestable vibration characteristics can be simplified or omitted, andproductivity can thus be increased.

It will be recognized that the present invention is not limited to thecombinations of differently shaped mask patterns used for thefabrication of the crystal devices 1 to 4 described above.

1. A crystal device having a crystal plate that includes a base and avibrating prong protruding from said base, comprising: a first majorface, which contains a portion of said base and a portion of saidvibrating prong within a single plane, and formed on said crystal plate;a second major face, which contains another portion of said base andanother portion of said vibrating prong within a single plane, formed onsaid crystal plate, wherein said first major face and said second majorface have different outer shapes.
 2. The crystal device according toclaim 1, wherein said crystal plate includes at least a first vibratingprong and a second vibrating prong.
 3. The crystal device according toclaim 2, wherein said first and second major faces each have a crotchportion defined by a line that originates from the tip of said firstvibrating prong, passes through the root of said first vibrating prongand through the root of said second vibrating prong, and leads to thetip of said second vibrating prong, and wherein the crotch portion ofsaid first major face and the crotch portion of said second major facehave different shapes.
 4. The crystal device according to claim 3,wherein the number of bends contained in the crotch portion of saidfirst major face is different from the number of bends contained in thecrotch portion of said second major face.
 5. The crystal deviceaccording to claim 3, wherein the angle of a bend contained in thecrotch portion of said first major face is different from the angle of abend contained in the crotch portion of said second major face.
 6. Thecrystal device according to claim 3, wherein the number of center pointsrepresenting curved sections contained in the crotch portion of saidfirst major face is different from the number of center pointsrepresenting curved sections contained in the crotch portion of saidsecond major face.
 7. The crystal device according to claim 3, whereinthe curvature of a curved section contained in the crotch portion ofsaid first major face is different from the curvature of a curvedsection contained in the crotch portion of said second major face.
 8. Amethod for manufacturing a crystal device having a crystal plate thatincludes a base and a vibrating prong protruding from said base, themethod comprising the steps of: forming a resist layer made of aphotosensitive material and a mask layer having corrosion resistance oneach of two plane surfaces of a crystal substrate; exposing the resistlayer formed on the first plane surface of said crystal substrate toradiation through a first mask on which a first mask pattern is drawn,and exposing the resist layer formed on the second plane surface of saidcrystal substrate to radiation through a second mask on which a secondmask pattern is drawn that differs in shape from said first maskpattern; forming a first resist layer pattern by patterning the resistlayer on the first plane surface of said crystal substrate into a shapecorresponding to said first mask pattern, and forming a second resistlayer pattern by patterning the resist layer on the second plane surfaceof said crystal substrate into a shape corresponding to said second maskpattern; forming a first mask layer pattern by patterning the mask layeron the first plane surface of said crystal substrate into the shapecorresponding to said first mask pattern, and forming a second masklayer pattern by patterning the mask layer on the second plane surfaceof said crystal substrate into the shape corresponding to said secondmask pattern; and forming said crystal plate by etching said crystalsubstrate through said first mask layer pattern and said second masklayer pattern.
 9. The method according to claim 8, wherein said firstmask layer pattern has an outer shape that contains a portion of saidbase and a portion of said vibrating prong within a single plane, andsaid second mask layer pattern has an outer shape that contains anotherportion of said base and another portion of said vibrating prong withina single plane.
 10. The method according to claim 8, wherein saidcrystal plate includes at least a first vibrating prong and a secondvibrating prong.
 11. The method according to claim 10, wherein saidfirst and second mask layer patterns each have a crotch portion definedby a line that originates from the tip of said first vibrating prong,passes through the root of said first vibrating prong and through theroot of said second vibrating prong, and leads to the tip of said secondvibrating prong.
 12. The method according to claim 11, wherein thecrotch portion of said first mask layer pattern and the crotch portionof said second mask layer pattern have different shapes.
 13. The methodaccording to claim 12, wherein the number of bends contained in thecrotch portion of said first mask layer pattern is different from thenumber of bends contained in the crotch portion of said second masklayer pattern.
 14. The method according to claim 12, wherein the angleof a bend contained in the crotch portion of said first mask layerpattern is different from the angle of a bend contained in the crotchportion of said second mask layer pattern.
 15. The method according toclaim 12, wherein the number of center points representing curvedsections contained in the crotch portion of said first mask layerpattern is different from the number of center points representingcurved sections contained in the crotch portion of said second masklayer pattern.
 16. The method according to claim 12, wherein thecurvature of a curved section contained in the crotch portion of saidfirst mask layer pattern is different from the curvature of a curvedsection contained in the crotch portion of said second mask layerpattern.